1. Field of the Invention
The present invention relates to a Radio Frequency Identification (RFID) tag and an apparatus and method for locating a RFID tag, and more particularly, to an apparatus and method for locating a RFID tag without comparing arrival times of blink signals transmitted from the RFID tag in order to quickly trace a location of the REID tag.
2. Description of Related Art
A Radio Frequency Identification (RFID) technology was introduced to recognize, locate, and identify a target object including a person and an animal in wireless manner. Lately, the RFID technology has been frequently used in daily life. For example, one of representative applications of the RFID technology is a barcode system or a magnetic card system. Such a RFID technology has been continuously advanced to satisfy various demands from users and to overcome shortcomings of a barcode system or a magnetic card system.
In general, a RFID card indicates a contactless card. Unlike a contact card, it is not necessary for a user to insert a RFID card into a reader. Accordingly, the RFID card is less damaged or contaminated by peripheral environment.
A RFID system includes a tag, a reader, and a data processing system for processing data read from the tag. The tag and the reader communicate with each other through a wireless link.
The tag internally includes a memory, an integrated circuit (IC), a microprocessor, and an antenna. The tag is classified into an active tag and a passive tag by an internal energy source.
The active tag includes an own power supply and the passive tag receives power from the reader through induction current. Accordingly, it is possible to locate the active tag from a comparative long distance compared to the passive tag.
The antenna of the reader continuously transmits a radio wave. When a tag storing ID and data enters a radio wave coverage of the antenna, the tag transmits the ID and stored data to the antenna.
A RFID system includes a RFID tag (transponder) for storing unique information, a RFID reader for reading and decoding data stored in the RFID tag, a host computer (server) for processing data read from the RFID tag, application software, and a network.
The RFID tag is also referred as a transponder which is a compound word of a transmitter and a responder. The RFID tag includes an IC chip and an antenna circuit. The RFID tag communicates with the reader through the antenna and a RF module.
The RFID tag is classified by a size, a shape, and a type of a memory for storing data. Particularly, the RFID tag can be classified into a read-only RFID tag, a read/write RFID tag, and a write once read many (WORM) RFID tag by a memory type.
As described above, the RFID tag is also classified into an active tag and a passive tag according to whether a RFID tag internally includes a power supply or not. In case of the active tag, the RFID tag transmits and receives data using the own power supply. In case of the passive tag, a RFID tag transmits and receives data through induction current from a reader.
Accordingly, it is possible to track the active tag even from a comparative long distance compared to the passive tag. However, the passive tag has advantages of light-weighted, low cost, and long lifespan. Accordingly, the passive tag is generally used when a tag is required to frequently communicate with a reader, when a tag is required to communicate with a reader for a long time, and when a tag is required to have no limitation to store data.
Since a passive tag has limitation in a distance, a system using an active RFID tag has been receiving attention. The active tag is disadvantageously required to include a power supply. On the contrary, the active tag is convenient to use and advantageously provides various services. The active tag can be designed in various types according to a frequency band and a use purpose.
A low frequency RFID system uses a frequency range of 30 kHz to 500 kHz and is used in a short transmission area such as 1.8 m. A radio frequency RFID system uses a frequency range of 850 MHz to 950 MHz or a frequency range of 2.4 GHz to 2.5 GHz and is a long distance transmission area such as 27 m.
Hereinafter, a method for locating an active RFID tag in a RFID system according to the related art will be described.
A Real-Time Location System (RTLS) is a system for locating a target object such as a person or an object in real time using a RFID and sensor technology and a network technology. That is, since the RTLS provides a real time service of locating a target object, the RTLS must calculate and confirm a location of a target object within 30 seconds after the RTLS receives blinking information from an active RFID tag.
The RTLS provides various services through interacting with many external systems. The most RTLSs in a market are an active RFID system using a wireless communication system based on 2.45 GHz frequency band.
The RTLS uses one of a triangulation based locating method and a finger printing based locating method to calculate a location of a tag. In general, the triangulation based locating method is used to locate a target object due to the advantages of triangulation such as convenience and easy operation of related equipment.
The triangulation based locating method includes two processing steps: measuring a distance and calculating a location through triangulation. Since the triangulation is well known algorithm, it is very easy to realize the process step of calculating a location if distances are given. Since various distance measuring methods have been introduced, it is very important to deeply understand the characteristics of each distance measuring method in order to carefully select one proper to a locating system.
Representative distance measuring methods are a received signal strength intensity (RSSI) based method using an intensity of a received signal, a time of flight (TOF) based method using a time taken to transfer a signal between devices, a time different of flight (TDOF) based method using a difference of flight times of radio waves, an angle of arrival (AOA) based method using an angle of arrival of radio wave, and a phase of arrival method using a phase of arrival of a radio wave. The RSSI based method, the TOA based method, and the TDOA based method have been widely used. The TDOA based method is also referred as a TOF based method and a TDOF based method.
Hereinafter, a method for locating a target object in a RTLS according to the related art will be described with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating a Direct Sequence Spread Spectrum (DSSS) signal transmitted from a RTLS tag. The DDSS signal is defined in ISO/IEC 24730-2.
As shown in FIG. 1, a RTLS tag transmits a blink signal modulated by the Direct Sequence Spread Spectrum (DSSS) scheme at a blink interval for example minimum 5 seconds. Each blink is formed of N sub blinks. Each sub blink is transmitted using one selected from four message types defined in ISO/IEC 24730-2. For example, each blink includes one to eight sub blinks. In order to obtain time diversity, the sub-blinks have a randomized sub-blink interval of 125 ms±16 ms.
A location processor engine calculates a location using a TDOA algorithm based on an arrival time of received sub blink from a RTLS reader. In order to calculate a location of a RTLS tag, the location processor engine needs at least three arrival times of sub blinks from readers. The location processor engine calculates a time difference of the arrival times of sub blinks using a TDOA algorithm. In order to accurately calculate, it is necessary to group sub blink arrival time information by the same transmission time among N sub blink transmissions. If they are wrongly grouped, a sub blink interval will have a time different error about of 125 ms±16 ms. If the location of a RTLS tag is calculated using this wrong information, it may cause error greater than 3 m which is proposed in ISO/IEC 24730-2.
FIG. 2A is a flowchart illustrating a method of locating a RTLS tag in a location processor engine when a plurality of RTLS tags transmit a blink signal according to an ISO/IEC 24730-2.
Each RTLS tag is an active REID having a unique ID. Each RTLS tag is attached to a target object such as an object or a person. The RTLS tag regularly transmits own information to a reader. At step 200, a location processor engine receives information about arrival times of sub blinks and tag ID values from a plurality of tags and sequentially groups the received information. At step S202, the received information is grouped by the same tag ID by comparing the tag ID values.
At step S204, a sub blink group is allocated by comparing current arrival time information with previous arrival time information in the allocated tag ID group. If there is not previous arrival time information because it is the first arrival time information, the first sub blink group is allocated.
At step S206, if a time difference of the previous arrival time and the current arrival time is shorter than about 10 μs, the current arrival time information is determined as arrival time information of a sub-blink transmitted at the same transmission time. At step S210, the same sub-blink group of the previous arrival time information is allocated to the current sub blink arrival time information.
10 μs is a sub-blink arrival time based on a speed of propagation of radio wave when a RTLS tag and a RTLS reader are separated at about 3 km. Further, since it is a further wider range of 300 m which is defined in ISO/IEC 24730-2 standard, 10 μs can be an enough threshold value to group sub-blink arrival time information.
If the time difference is greater than 10 μs at step 2208, the current arrival time information is determined as arrival time information of sub-blink not transmitted at the same transmission time. Therefore, a next sub-blink group is allocated to.
At step S212, the arrival time information and the tag ID values from readers are grouped by repeating the above processes. After finishing grouping the received information, the information is group as shown in FIG. 2B. As shown in FIG. 2B, the tag ID-grouped information Gtagi 222 includes sub-blink grouped information Gsubi 224. The tag ID-grouped information Gtagi 222 corresponds to the number of RTLS tags used to locate a RTLS tag. The sub-blink grouped information Gsubi 224 corresponds to the number of sub-blink arrival time information received from reader. The tag ID-grouped information Gtagi 222 may include one to eight sub-blink grouped information Gsubi 224.
After finishing grouping, it is determined whether the sub-blink arrival time information is more than three or not at step S214. If it is more than three, a location of the RTLS tag is calculated using a TDOA algorithm at step S218. At step S220, a coordinate and a corresponding tag ID of the calculated RTLS tag is transmitted to a server.
If the number of sub-blink arrival time information is less than three, it is impossible to calculate a location of the RTLS tag. Accordingly, a location of the RTLS tag is not calculated and corresponding data is dumped at step S216.
However, the method of locating a RTLS tag in a RTLS system according to the related art requires additional algorithm to group sub-blink arrival time information which is required to calculate a location of a RTLS tag in a location processor engine. Further, it is necessary to compare the sub-blink arrival time information through the predetermined algorithm. Accordingly, the method of locating a RTLS tag according to the related art is inefficient.